JPH0812388A - Production of cement clinker - Google Patents

Abstract

PURPOSE:To produce a cement clinker capable of showing excellent color tone and mechanical strength by recovering the quality of a cement clinker adversely affected due to firing in a reducing atmosphere. CONSTITUTION:The cement clinker exposed to the reducing atmosphere in a kiln is charged into an auxiliary kiln kept in an oxidizing atmosphere or again into the kiln and is fired in an oxidizing atmosphere, e.g. the atmosphere of >=1.0% oxygen concn., at >=130 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for modifying cement clinker which has an undesired quality because it is produced in a reducing atmosphere.

[0002]

2. Description of the Related Art In recent years, it has been actively studied to put industrial waste such as waste tires or raw garbage into a rotary kiln and use them as raw fuel. However, if they are added near the downstream side of the kiln, the cement clinker may come into direct contact with the burning industrial waste or garbage and be burned in a reducing atmosphere.

When cement clinker is fired in a reducing atmosphere, the valence of ions easily changes due to changes in the oxygen concentration in the high temperature firing atmosphere of Fe, Mn, etc. in the cement raw material, preventing the formation of necessary minerals, It may reduce the strength development of the cement or change the color development state to give an undesired color tone. Specifically, when Fe is exposed to a reducing atmosphere, the valence of ions increases from 3+ to 2+, 0.
It becomes + and changes as Fe ₂ O ₃ → FeO → Fe (metal). Thus, to produce a cement clinker in a car that has been fired in an oxidizing atmosphere _{4CaO · Al 2 O 3 · Fe} 2 O 3 is the formula _{4CaO · Al 2 O 3 · Fe} 2 O 3 → 3CaO · Al
_It reacts like ₂ O ₃ + CaO + FeO and Fe, and part of the produced FeO dissolves in 2CaO.SiO ₂ and part of it stays in the interstitial layer. Therefore, unbound C in the clinker
aO increases and strength development decreases.

Further, black mineral is reduced by a reduction in _{4CaO · Al 2 O 3 · Fe} 2 O 3, which ground cement exhibits a color tone of undesirable reddish brown.

The degree of influence of the above-mentioned reducing atmosphere is Fe, M
It varies depending on conditions such as the amount of metal ions whose ionic valences such as n and Cr are easily changed, oxygen concentration, carbon monoxide concentration, and temperature.

[0006] These phenomena occur regardless of the method of firing the cement clinker. As the model of the kiln,
It occurs in the firing method such as the latest rotary kiln with new suspension preheater, fluidized bed kiln, etc., which directly contacts fuel with clinker raw material.

[0007]

Means for Solving the Problems Accordingly, the present inventors have:
We have earnestly studied how to remove the undesired effects that appear in the cement clinker fired in the reducing atmosphere. As a result, even if the clinker was burned in a reducing atmosphere, the metal ions in the clinker could be oxidized by firing again in a sufficient oxygen atmosphere and temperature, and the clinker had the same quality as the clinker fired in the original oxidizing atmosphere. They have found that they can be recovered and have completed the present invention.

That is, according to the present invention, the cement clinker exposed to a reducing atmosphere is put into an auxiliary firing furnace kept in an oxidizing atmosphere, or is put into the firing furnace again to be fired in an oxidizing atmosphere. And a method for producing a cement clinker.

In the firing furnace, the cement raw material is originally fired in an oxidizing atmosphere to form a cement clinker having a normal mineral composition. However, as described above, when industrial wastes are burned as fuel in a firing furnace, the surroundings become a reducing atmosphere, which hinders the normal reaction that occurs in an oxidizing atmosphere and causes cement clinker with an abnormal mineral composition. can get. For this reason, the mechanical strength of the obtained cement mortar is lowered, or the color tone of the cement clinker is dark green to
It is out of the normal color of black and becomes an abnormal color of reddish brown to reddish yellow.

In order to determine whether or not the cement clinker is exposed to a reducing atmosphere in the firing furnace, the cement clinker discharged from the firing furnace is sampled, and the cement clinker having a normal tone is visually inspected or using a color difference meter. It can be done by comparing.

In order to fire the cement clinker exposed in the reducing atmosphere in the firing furnace in the oxidizing atmosphere, the cement clinker is put in the auxiliary firing furnace held in the oxidizing atmosphere or again in the firing furnace. Here, as a method of firing in an oxidizing atmosphere, a method of preparing an auxiliary firing furnace different from the firing furnace and charging the cement clinker discharged from the firing furnace into the auxiliary firing furnace, or a furnace other than the firing furnace It is possible to use a method in which the cement clinker discharged from the firing furnace is charged again from the inlet of the firing furnace without using the above.

Known furnaces can be used as the firing furnace and the auxiliary firing furnace. For example, as a firing furnace,
Examples include rotary kilns and fluidized bed kilns. The auxiliary firing furnace is a furnace different from the firing furnace. The firing furnace and the auxiliary firing furnace may use the same type of furnace in combination, or may use different types of furnaces.

The firing temperature in the oxidizing atmosphere is set so that 3CaO.SiO ₂ in the cement clinker exposed to the reducing atmosphere is not decomposed into 2CaO.SiO ₂ + CaO.
It is preferably 00 ° C or higher, and further 1400 ° C
The above is more preferable. The upper limit of the temperature is not particularly limited, but it is generally preferably 1500 ° C. or lower in order to avoid wasteful consumption of energy.

Generally, oxygen and carbon monoxide coexist in the firing atmosphere, and when the oxygen concentration in the firing furnace outlet gas is less than 1%, the carbon monoxide concentration is 0.1%.
In some cases, a sufficient oxidizing atmosphere cannot be formed in excess of 3%. Therefore, the oxygen concentration in the oxidizing atmosphere is always 1
% Or more, more preferably 1.5% or more.

The firing time in an oxidizing atmosphere varies depending on the firing zone temperature, but it is sufficient to perform firing for about 10 minutes or longer at a firing temperature of 1400 ° C. or higher and an oxygen concentration in the firing furnace outlet gas of 1.5% or higher. It is possible to obtain various oxidation and firing effects.

[0016] It is preferable that the cement clinker has a small particle size, that is, a large specific surface area in firing in an oxidizing atmosphere, but since a general clinker is sufficiently porous,
From the viewpoint of energy effect, it is sufficient to pulverize up to about 5 mm sieve. Sufficient conditions for re-baking, eg 1
If the temperature is 400 ° C. or higher, the oxygen concentration is 1.5% or higher, and the time is 10 minutes or longer, there is no problem even if re-baking is performed without crushing.

The clinker recalcined under such conditions is in an oxidatively calcined state, and this clinker is
By quenching from a temperature of 250 ° C or higher, high temperature stable minerals of clinker M1, M3-3CaO · Si
O ₂ and α-, β-2CaO · SiO ₂ can be stabilized at room temperature. As the method of quenching, any method such as air cooling as in a grade cooler or cooling with water spray may be used, and it is desirable to cool from 1250 ° C. to 500 ° C. or less in about 10 minutes.

The fuel for re-firing in an oxidizing atmosphere is not particularly limited, but a fuel having a sufficiently high combustibility so that the clinker surface is not exposed to a reducing atmosphere,
For example, heavy oil and pulverized coal are desirable.

In the re-baking, if the amount of heat held by the clinker is completely removed and then the clinker is heated again, the energy efficiency becomes poor. Therefore, it is desirable to re-fire without lowering the clinker temperature as much as possible. That is, when it is known that the clinker is obviously exposed to the reducing atmosphere, it is desirable to re-fire the clinker discharged from the firing furnace as it is without guiding it to the cooler.

In this way, by recalcining the cement clinker exposed to the reducing atmosphere in the oxidizing atmosphere, it is possible to manufacture cement that is not affected by the reducing atmosphere.

[0021]

EFFECTS OF THE INVENTION According to the method of the present invention, it is possible to prevent the quality of the clinker from being adversely affected by the baking under the reducing atmosphere. That is, since the strength of the cement is not lowered and the color tone is not changed, it can be used from general construction work to secondary concrete products without any problem.

[0022]

EXAMPLES The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

Comparative Examples 1 to 3 Using a rotary kiln with a new suspension preheater (Nissan 6000t), the clinker for ordinary Portland cement having the components shown in Table 1 was put in a kiln butt oxygen concentration of 1.5.
% And CO concentration of 0.2% or less. The oxidizing atmosphere-baked clinker thus obtained was used as Comparative Example 1.

Further, the oxygen concentration in the firing furnace outlet gas is 0.5
Comparative Example 2 was a calcined clinker in a relatively weak reducing atmosphere having a CO concentration of 0.5% or less and a CO concentration of 0.5% or more.

Further, as shown in FIG. 1 of the above rotary kiln, a plastic combustion burner was attached, and the plastic was injected as a fuel under the conditions shown in Table 2. In this case, the plastic burned in contact with the cement clinker and entered the cooling zone while keeping the clinker surface in a strongly reducing atmosphere. The strongly reduced calcined clinker in this case was set as Comparative Example 3.

[0026]

[Table 1]

[0027]

[Table 2]

Examples 1 to 3 After cooling the cement clinker obtained in the above Comparative Examples 2 and 3, the cement clinker was charged again from the kiln bottom of the rotary kiln, and pulverized coal having sufficiently fast combustibility was used, and the conditions shown in Table 2 were used. It was baked in. These are Examples 1 and 2, respectively.

Further, the clinker of Comparative Example 3 was ground to a size of 5 mm through the sieve and then re-fired in an electric furnace at 1400 ° C. for 2 minutes at an oxygen concentration of 21%. This was designated as Example 3.

With respect to the clinker of Comparative Examples 1 to 3 and Examples 1 to 3 thus obtained, 3 parts by weight of gypsum was added to 100 parts by weight of the clinker, and the specific surface area was 3200 c.
The cement obtained by pulverizing with a ball mill to m ² / g was subjected to a physical test according to JIS R5201, measurement of the amount of unreacted calcium according to the Japan Cement Association method (I-01), and cement with NBS colorimetry. The color was measured and the produced mineral was identified by the X-ray diffraction method. The results are shown in Table 3.

From these results, the quality of the reduction-baked clinker subjected to reoxidation baking in the rotary kiln and the electric furnace was restored to a quality almost equal to that of Comparative Example 1.

[0032]

[Table 3]

[Brief description of drawings]

FIG. 1 is a schematic view of a rotary kiln used in a comparative example.

[Explanation of symbols]

 1 Rotary Kiln 2 Upstream End 3 Plastic 4 Downstream End 5 Plastic Injection Nozzle 6 Main Fuel Nozzle 7 Plastic Attainment Area 8 Plastic Combustion Area 9 Oxidizing Atmosphere Firing Zone

Claims (1)

[Claims] 1. A cement clinker exposed to a reducing atmosphere in a firing furnace is put into an auxiliary firing furnace maintained in an oxidizing atmosphere, or is put into the firing furnace again and fired in an oxidizing atmosphere. Method for producing cement clinker.